This invention relates to NTC (negative temperature coefficient) thermistors, and more particularly to NTC thermistors of the so-called film type having a temperature-sensitive film formed on an electrically insulating substrate by a thin-film or thick-film forming process.
NTC thermistors are widely being used for the purposes of temperature detection and temperature compensation. Japanese Patent Publication Tokkai 64-50501, for example, disclosed an NTC thermistor of a film type having a temperature-sensitive film formed on an electrically insulating substrate by a process of high-frequency sputtering in order to improve the response characteristic when used for purposes such as temperature detection.
FIG. 6 shows an example of prior art NTC thermistor of a film type having an electrically insulating substrate 51 (herein referred to simply as the xe2x80x9csubstratexe2x80x9d) made of an electrically insulating material, a temperature-sensitive film (xe2x80x9cthermistor filmxe2x80x9d) 52 made of a thermistor material on the surface of the substrate 51 and a pair of electrodes (xe2x80x9csurface electrodesxe2x80x9d) 53 formed on the surface of the temperature-sensitive film 52. Compared to an NTC thermistor of a so-called bulk type characterized as having a thermistor element for temperature detection with electrodes formed on a thermistor member entirely made of a thermistor material, such an NTC thermistor of a film type is advantageous not only wherein only a smaller amount of a thermistor material is required but also wherein the response time, which is the most important characteristic of an NTC thermistor for temperature detection, can be significantly improved because its thermal capacity is small.
For making prior art NTC thermistors of the film type, however, thermistor materials including oxides of transition metals such as Mn, Ni, Co, Fe and Cu which are used for the production of NTC thermistors of a bulk type were used for their temperature-sensitive film. Such materials usually have a specific resistance equal to or greater than 500 xcexa9cm. NTC thermistors of a bulk type having a desired resistance value can be obtained with such a thermistor material, but if such a thermistor material is used for the production of an NTC thermistor of a film type as shown in FIG. 6, its resistance value becomes much higher than that of a prior art bulk type NTC thermistor.
In view of this problem, aforementioned Japanese Patent Publication Tokkai 64-50501 disclosed a method of oxidizing the film, after it is formed by a high-frequency sputtering method, by a plasma process inside an atmosphere of an oxidizing gas. This production method is disadvantageous because an extra equipment is required for carrying out the plasma processing after the film is formed and the production process as a whole takes a much longer time.
Japanese Patent Publication 63-266801 disclosed a method of providing electrodes on both upper and lower surfaces of a temperature-sensitive resistor film. With a thermistor thus structured, however, since the thickness of the temperature-sensitive resistor film is extremely small and the distance by which the electrodes are separated from each other is the same as this film thickness, troubles such as short-circuiting are more likely to occur.
It is therefore an object of this invention, in view of the situation described above, to provide a reliable NTC thermistor with a fast response speed in temperature detection and a low resistance value.
An NTC thermistor embodying this invention, with which the above and other objects can be accomplished, may be characterized as comprising an electrically insulating substrate, a temperature-sensitive film on a surface of the substrate containing oxide of rare earth elements such as LaCoO3 as its principal component by at least 50 weight %, and a pair of electrodes which are separated from each other and are each electrically connected to this film. An NTC thermistor chip embodying this invention can be obtained therefrom by further forming a pair of outer electrodes which are each on a corresponding one of the end portions and electrically connected to a corresponding one of the surface electrodes.
By thus forming an NTC thermistor of a film type, the speed of response can be improved significantly over the prior art NTC thermistors of the bulk type using oxides of transition metals such as Mn, Ni, Co, Fe and Cu while the resistance value can be as low as that of such a bulk type NTC thermistor. Moreover, NTC thermistors according to this invention are found to be reliable in that the fractional change in the resistance value by a shelf test can be made small.